Inhibition of corrosion of hydraulic fluids



3,331,779 Patented July 18, 19 67 3,331,779 INHIBITION OF CORROSION F HYDRAULIC FLUIDS Donald H. Nail, Los Angeles, Calif., assignor to Douglas Aircraft Company, Inc, Santa Monica, Calif., a corporation of Delaware No Drawing. Filed Dec. 30, 1963, Ser. No. 334,580 12 Claims. (Cl. 252-78) This invention relates to compositions and to hydraulic fluids containing phosphinates, and is particularly concerned with improved phosphinate-containing compositions and hydraulic fluids which are inhibited against corrosion of metals, particularly iron.

In the copending application of Robert S. McCord, Ser. No. 334,588, filed Dec. 30, 1963, is disclosed novel hydraulic fluids containing a phosphinate as a major component, and designed for operation at high temperatures in the hydraulic systems of modern aircraft, particularly supersonic aircraft. Such hydraulic fluids have many advantages over prior art hydraulic fluids employed in hydraulic systems of aircraft, particularly high stability and high fire resistance. Although the phosphinate component, specifically the aryl dialkyl phosphinate, of the hydraulic fluids described in the above copending application, or mixtures of such phosphinate with thickeners, such as described in the above application, and forming the improved hydraulic fluids thereof, are suitable for use with and are substantially non-corrosive to many metals and alloys such as aluminum, titanium, silver and certain types of steels, e.g., 18-8 stainless steels, such phosphinates or mixtures thereof with thickeners corrode certain metals such as low carbon steels. Such low carbon steels are exemplary of the types of steel often used for construction of pumps employed in hydraulic systems of aircraft.

It is accordingly an object of the invention to inhibit corrosion of metals such as iron by compositions containing organic phosphinates.

Another object is to provide hydraulic fluids containing organic phosphinates as a major component, and which have substantially reduced corrosiveness on metals such as iron, especially low carbon steels, and particularly at high temperatures, e.g., of the order of about 400 F.

Another object of the invention is the provision of improved hydraulic fluids containing an aryl dialkyl phosphinate as a major component, and a suitable thickener, and inhibited against attack of metals such as iron, particularly low carbon steels, by incorporation of an effective inhibitor.

A still further object of the invention is to provide novel hydraulic fluids containing an aryl dialkyl phosphinate, preferably phenyl di n-butyl phosphinate, as a major component, and a suitable thickener, e.g., a polyalkyl methacrylate, effective to increase substantially the viscosity index of said phosphinate, and providing a thermally stable and fire-resistant hydraulic fluid operable at high temperatures in the range of about 400 to about 550 F., said hydraulic fluids having an additive incorporated therein inhibiting or substantially reducing attack of said hydraulic fluids on iron, particularly low carbon steels, particularly at high temperatures of the order of about 400 F., preferably without adversely affecting the properties of such hydraulic fluids.

Other objects and advantages will be in part apparent and in part obvious from the following description of the invention.

The term iron employed in the specification and claims, unless otherwise indicated, is intended to denote and include iron and steels, particularly low carbon steels.

I have found that by addition of a small or minor amount of the amino compound N,N',N",N"-tetra-sali cylidene-tetra(amino-methyl)methane having the formula to an organic phosphinate, particularly an aryl dialkyl phosphinate, or to a composition containing such phosphinate in admixture with other materials, the resulting phosphinate material or composition containing the same, unexpectedly exhibits substantially reduced corrosion on iron, particularly low carbon steels, as compared to the phosphinate or phosphinate-containing composition in the absence of such additive. The invention principles are particularly applicable to the phophinate-containing mixtures forming the hydraulic fluids described in the above copending application, preferably employing an aryl dialkyl phosphinate as a major component and including a minor proportion of a suitable thickener. Of particular significance, such corrosion inhibition is attained substantially without adversely affecting the properties of the phosphinate-containing material and particularly the important properties of the above-noted hydraulic fluids, in the absence of such additive.

The hydraulic fluids described and claimed in the above copending application contain monoaryl dialkyl phosphinates having the general formula Where R is aryl, e.g., phenyl or naphthyl, and R is an alkyl group of from 4 to 8 carbon atoms, preferably an alkyl group of from 4 to 6 carbon atoms. Thus, the alkyl groups can be butyl, pentyl, hexyl, heptyl and octyl. Also, branched chain alkyl groups can be employed. In this respect, aryl dialkyl phosphinates can be employed wherein the alkyl groups are secondary and tertiary butyl, neopentyl, branched chain hexyls and heptyls, and also branched chain octyl groups, such as isooctyl.

Preferably, aryl dialkyl phosphinates are employed wherein the alkyl groups are straight-chain alkyls. The two alkyl groups in the phosphinates may be the same or different.

In the process of manufacture the above monoaryl diakyl phosphinates, the final reaction product contains, in addition to the desired aryl dialkyl phosphinates, small amounts of other side reaction products, including phosphinates and phosphine oxides. Thus, for example, in the preparation of phenyl di n-butyl phosphinate for use in hydraulic fluids, according to the invention, by practice of the above process a mixture of products is formed composed of about 70% to of the phenyl di n-butyl phosphinate, about 10% to about 15% of diphenyl nbutane phosphonate, and about 10% to about 15% of tri n-butyl phosphine oxide. This impure material itself is substantially as suitable as the purified aryl dialkyl phosphinate. To obtain a purer material, the above mixture may be distilled to increase the amount of phosphinate and reduce the amounts of the above-noted sidereaction products. Thus, the above-described mixture may be distilled to obtain a composition composed of about 87% of the phenyl di n-butyl phosphinate, about 7% diphenyl n-butane phosphonate, and about 6% tri n-butyl phosphine oxide. Hence, the term aryl dialkyl phosphinate employed herein and in the claims is in.- tended to denote either the pure compound or an impure mixture containing a major proportion of aryl dialkyl phosphinate.

If desired, a single one of the above-defined phosphinates can be employed in the hydraulic fluid or a mixture of such phosphinates can be employed. The preferred aryl dialkyl phosphinate is phenyl di n-butyl phosphinate.

According to the above application, a minor amount of a compatible thickener or thickening agent is employed in conjunction with the aryl dialkyl phosphinates in order to provide a hydraulic fluid having proper viscosity characteristics at hydraulic fluid operating temperatures of the order of 400 to 550 F., a minimum increase in viscosity at low temperature and a high viscosity index. Thus, such thickener should be effective to increase the viscosity index of the phosphinate-containing hydraulic fluid to at least 75, e.g., in the range of about 75 to about 150, with the resulting fluid containing the thickener having a viscosity at low temperature down to about 40 F. not in excess of about 14,000 centistrokes. The thickener should also have high thermal stability at temperature of at least 400 F., and provide a hydraulic fluid having high fire resistance. Examples of suitable thickeners for this purpose include the polyalkyl methacrylates, the polyalkylene glycols, the polyurethanes and the methyl phenyl silicone polymers, in the order of performance named.

The polyalkyl methacrylates employed are generally those resulting from the polymerization of alkyl methacrylates in which the alkyl groups can have an average of from about 3 to 10 carbon atoms. Preferably, the alkyl groups of the methacrylate can vary from about 4 to about 8 carbon atoms. The poly n-butyl methacrylate is preferred for use with the preferred phosphinate material, i.e., phenyl dibutyl phosphinate. The average molecular weight of the polyalkyl methacrylate can range from about 7,000 to about 12,000. Another suitable thickener are the high molecular weight polyalkylene glycols. Suitable materials of this type are those in which the alkylene groups contain from 2 to 3 carbon atoms. Thus, specific examples of such polyalkylene glycols are the high molecular weight polypropylene glycols, polyisopropylene glycols, and copolymers such as the ethylene glycol, isopropylene glycol copolymer. Such high molecular Weight polyalkylene glycols are characterized by having a range of viscosity of about 1,400 to about 23,000 centistokes at 100 F. Also suitable as thickeners are the urethane polymer liquids. These may be urethane polyether or urethane polyester materials. Another thickener which can be added to the above aryl dialkyl phosphinate are the liquid methyl phenyl silicone polymers, preferably of high molecular weight, and characterized by having a viscosity, e.g., in the range of about 450 to about 575 centistokes at 100 F. The polymers of this type most useful in the hydraulic fluids of the above application are those having a high phenyl to methyl ratio, e.g., of at least about 1:1, and which may be about 2:1, or higher.

Usually a minor proportion, and generally from about 0.5% to about 10% by weight of the thickener, based on the Weight of the total composition, produces the desired eflectiveness in the aryl dialkyl phosphinate base stock.

In preferred practice, a portion of about 1% to about by weight of thickener is employed.

The entire description of the phosphinates and the hydraulic fluids containing such phosphinates in the abovenoted copending application is incorporated herein by reference.

The development of suitable inhibitors against attack of the above-described phosphinates on iron, and which are compatible and suitable for use With such phosphinates, and particularly for incorporation in the abovedescribed hydraulic fluids containing such phosphinates as an essential component, without adversely affecting the properties of such materials, has been complex. The difficulties in developing suitable additives for this purpose, and particularly for use with such hydraulic fluids, are the requirements that such additives must be compatible with or soluble in such phosphinate-containing compositions or hydraulic fluids over a broad temperature range, the additives should not cause undesirable gelation of these compositions tending, for example, to transform such compositions or fluids into a thixothropic material, and should not have any adverse side effects or side reactions on the components of these compositions, particularly the organic phosphinates, specifically the aryl dialkyl phosphinate component thereof. Also, the inhibitor should not operate to increase corrosion on the metals which are not normally affected by the phosphinate-containing material, and also the volatility of the additive should be low enough so that the inhibitor is not volatilized at the higher temperatures of operation. Further, for preferred use in the above-described hydraulic fluids which are intended for operation at high temperatures in the range of 400 to 550 F., it is necessary that such additives have good thermal stability at temperature of at least about 400 F., preferably in the range of about 400 to about 550 F., and fire resistance.

The N,N',N",N"' tetra salicylidene tetra-(aminomethyl)methane additive, in addition to possessing high inhibiting effectiveness of the attack of the above organic phosphinates on iron, also possesses substantially all of the above characteristics.

The amount of the N,N',N",N"-tetra-salicylidenetetra(amino-methyDmethane additive incorporated into the organic phosphinate-containing material, preferably the above-described hydraulic fluids, to achieve the abovedescribed inhibiting effectiveness, can range from about 0.001% to about 1%, preferably from about 0.001% to about 0.10% by Weight of the composition. The use of about 0.01% to about 0.04% by Weight of such additive has been found particularly satisfactory.

Although an amount of additive greater than 1% can be employed, Where such amount of additive is soluble in the base phosphinate material, such larger amounts usually do not materially enhance the effectiveness of the additive. Amounts of additive smaller than 0.001% by Weight generally are of reduced effectiveness in producing the desired inhibition of corrosion according to the invention.

The following are examples of practice according to the invention:

EXAMPLE 1 The following compositions were prepared by dissolving in each case the indicated amount of inhibitor in the indicated amount of phosphinate. In the examples below, the term additive is intended to denote N,N,N",N'- tetra-salicylidene-tetra amino-methyl methane.

Composition A Percent by Weight Each of Compositions A, B and C, and a control consisting of phenyl di n-butyl phosphinate alone in the absence of any inhibitor, was tested for corrosion on iron in the form of a low carbon steel, specifically a mild carbon steel meeting the requirements of Federal specification QQ-S-636, copper, aluminum, titanium and silver, metals often used in construction of a hydraulic system for modern aircraft, as follows: 1" x 1" x .032" thick samples of each of these metals were arranged in layers strung on a fine iron wire and separated by stainless steel washers, the washers separating the specimens sufliciently for the fluid being tested to reach their surfaces while the iron wire holds the bundle together in electrical contact as occurs in use. A separate bundle of such specimens was immersed in each of the fluid Compositions A, B, C and the control, and such fluids heated to 400 F. for 24 hours. The bundles of specimens were then removed from the fluids and cooled, and the change in appearance of the fluid and the respective metal samples, and the weight loss of each metal sample determined.

The results of these tests showed that substantial corrosion of the iron sample occurred in the control fluid not containing the invention inhibitor. Thus, the iron sample immersed in such fluid had a weight loss of about 6 mg./cm. and the appearance of the surface of the sample was rough and discolored. The control fluid had changed in appearance to an opaque black sediment-containing fluid due to such corrosion of the iron sample. A minor amount of corrosion occurred on the copper sample contacted with the control fluid. However, the aluminum, titanium and silver samples contacted with the control fluid remained substantially uncorroded.

On the other hand, the tests showed that the iron samples contacted with fluid Compositions A, B and C containing the invention inhibitor or additive had substantially reduced corrosion following the above treatment of such samples in these fluids. Weight loss of the iron samples contacted with Compositions A, B and C averaged about 2.5 mg./cm. showing that corrosion of the iron samples contacted with Compositions A, B and C was substantially reduced as compared to the corrosion of the iron sample contacted with the control fluid as described above. Fluid Compositions A, B and C following treatment, had a light pink coloration but were essentially clear in appearance, with substantially no sludge formation visible. The aluminum, titanium and silver samples contacted with Compositions A, B and C fluids remained uncorroded, showing that the invention inhibitor has substantially no adverse effect with respect to corrosion of such metals.

EXAMPLE 2 The following Compositions E, F, G and H were prepared by dissolving in each case the indicated amount of inhibitor in the indicated amount of phosphinate and polyalkyl methacrylate thickener forming a hydraulic fluid, Composition D being the control.

Composition D Percent by weight Phenyl di n-butyl phosphinate 94 A mixture of 30% by weight of a poly nbutyl methacrylate having an average molecular weight of 7,900, and 70% by weight of an organic phosphate ester as a carrier and solvent, marketed as Acryloid 6 CompositionG Phenyl di n-butyl phosphinate 93.96

Acryloid R3876X 6.00

Additive 0.04

Each of the hydraulic fluid Compositions D, E, F, and G was tested for corrosion on iron, copper, aluminum, titanium and silver, in the same manner and by the same procedure as set forth in Example 1 above.

The results obtained were similar to those obtained in Example 1. Thus, Composition D, the control, containing no inhibitor according to the invention, caused substantial corrosion of the iron sample, practically to the same extent as did the control in Example 1, whereas Compositions E, F and G, hydraulic fluids containing phosphinate, thickener and invention inhibitor, substantially reduced corrosion on the iron samples to about the same extent as did Compositions A, B and C, respectively, of Example 1. Similarly, as in the case of Example 1, Composition D, the control, and also the inhibited hydraulic fluids, Compositions E, F and G, caused essentially no corrosion of the aluminum, titanium and silver specimens.

Hence, it is apparent that the presence of the thickener in the phosphinate-containing hydraulic fluids of Compositions E, F, and G has substantially no affect on the function of the invention inhibitors in the inhibited hydraulic fluids of the invention for reducing corrosion on iron.

EXAMPLE 3 The procedures of Examples 1 and2 are repeated, but employing in each of Compositions A, B, C, E, F and G about 0.10% of the inhibitor.

Although results are obtained similar to those of Examples 1 and 2, the use of the smaller amounts of the in. hibitor or additive as in Examples 1 atnd 2 appears to product approximately equivalent inhibition results as compared to the use of the larger amount of 0.10% of the inhibitor.

EXAMPLE 4 The following are additional examples of inhibited phosphinate-containing compositions according to the invention.

Composition H Percent by weight Phenyl di n-pentyl phosphinate 96.95 A poly n-hexyl methacrylate having an average mo- Composition Percent by weight Phenyl di n-butyl phosphinate 94.8 A poly n-butyl methacrylate of molecular weight about 8,000 5.0 Additive 0.2

Composition P Naphthyl di neopentyl phosphinate 93.94 A poly n-butyl methacrylate having an average molecular weight of about 8,000 6.00 Additive 0.06

From the foregoing, it is seen that the invention provides phosphinate-containing compositions and valuable phosphinate-containing hydraulic fluids which are inhibited against attack of iron, particularly low carbon steels, important metals of construction of components of aircraft hydraulic systems, and which metals are subject to substantial corrosion in the presence of such uninhibited phosphinate-containing compositions and hydraulic fluids.

While I have described particular embodiments of my invention for the purpose of illustration, it should be understood that various modifications and adaptations thereof may be made within the spirit of the invention, as set forth in the appended claims.

I claim:

1. A composition consisting essentially of an aryl dialkyl phosphinate having the formula where R is an aryl group selected from the class consisting of phenyl and naphthyl, and R is an alkyl group containing from 4 to 8 carbon atoms, and about 0.001% to about 1% of N,N',N,N"-tetra-salicylidene-tetra(aminomethyl)methane by weight of said composition.

2. A composition as defined in claim 1, employing about 0.001% to about 0.10% by weight of said N,N',N",N"- tetra-salicylidene-tetra amino-methyl methane.

3. A composition as defined in claim 1, wherein said phosphinate is phenyl di n-butyl phosphinate.

4. A composition consisting essentially of an aryl dialkyl phosphinate having the formula where R is an aryl group selected from the class consisting of phenyl and naphthyl, and R is an alkyl group containing from 4 to 8 carbon atoms, and from about 0.5% to about 10% by weight of a material selected from the group consisting of polyalkyl methacrylates wherein the alkyl groups contain an average of from about 3 to 10 carbon atoms, and having an average molecular weight in the range from about 7,000 to about 12,000, polyalkylene glycols, wherein the alkylene groups contain from 2 to 3 carbon atoms, and having a range of viscosity of about 1,400 to about 23,000 centistokes at F., urethane polymer liquids, and liquid methyl phenyl silicone polymers having a viscosity in the range of about 450 to about 575 centistokes at 100 F., said material being compatible with said phosphinate and efi'ective to increase the viscosity index to at least 75, and about 0.001% to about 1% of N,N',N",N"'-tetra-salicylidene-tetra- (amino-methyl)methane by weight of said composition.

5. A composition as defined in claim 4, wherein said material is said polyalkyl methacrylate.

6. A composition as defined in claim 4, wherein said material is said polyalkylene glycol.

7. A composition as defined in claim 4, employing about 0.001% to about 0.10% by weight of said N,N,N",N" tetra salicylidene tetra(amino methyl) methane.

8. A composition as defined in claim 4, wherein said phosphinate is phenyl di n-butyl phosphinate.

9. A fire-resistant hydraulic fluid consisting essentially of phenyl di n-butyl phosphinate, about 0.5% to about 10% by weight of polybutyl methacrylate and having an average molecular weight in the range from about 7,000 to about 12,000, and about 0.001% to about 0.10% by weight of N,N,N",N"-tetra-salicylidene-tetra(aminomethyl)methane.

10. A fire-resistant hydraulic fluid consisting essentially of phenyl di n-butyl phosphinate, about 0.5% to about 10% by weight of a polyalkylene glycol wherein the alkylene groups contain from 2 to 3 carbon atoms, and having a range of viscosity of about 1,400 to about 23,000 centistokes at 100 F., and about 0.001% to about 0.10% by weight of N,N,N",N"-tetra-salicylidene-tetra(aminomethyl)methane.

11. A fire-resistant hydraulic fluid consisting essentially of phenyl di n-butyl phosphinate, about 0.5 to about 10% by weight of a urethane polymer liquid, and about 0.001% to about 0.10% by weight of N,N,N,N"-tetrasalicylidene-tetra(amino-methyl)methane.

12. A fire-resistant hydraulic fluid consisting essentially of phenyl di n-butyl phosphinate, about 0.5% to about 10% by weight of a liquid methyl phenyl silicone polymer having a viscosity in the range of about 450 to about 575 centistokes at 100 F., and about 0.001% to about 0.10% by weight of N,N,N",N'"-tetra-salicylidenetetra(amino-methyl)methane.

References Cited UNITED STATES PATENTS 2,174,019 9/1939 Sullivan 252--78 X 2,636,862 4/1953 Watson 25278 2,947,699 8/1960 Wasson et al. 252-76 LEON D. ROSDOL, Primary Examiner.

S. D. SCHWARTZ, R. D. LOVERING,

Assistant Examiners, 

4. A COMPOSITION CONSISTING ESSENTIALLY OF AN ARYL DIALKYL PHOSPHINATE HAVING THE FORMULA 